<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style type="text/css">
        .box {
            width: 200px;
            height: 200px;
            border: 1px solid red;
            position: relative;
        }
        .children-box {
            position: absolute;
            width: 100px;
            height: 100px;
            background: yellow;
            left: 50%;
            top: 50%;
            margin-left: -50px;
            margin-top: -50px; 
        }
        </style>
        
</head>
<body>
    <template>
        <div id="app">
            <div class="box">
                <div class="children-box"></div>
            </div>
        </div>
    </template>
</body>
</html>